1. Field of the Invention
The present invention relates to a color cathode ray tube that is used preferably as a television receiver or a computer display.
2. Description of Related Art
In a color cathode ray tube, electron beams emitted from an electron gun pass through apertures formed in a shadow mask, and then strike a phosphor screen, thus causing a phosphor to emit light.
As shown in FIG. 15, a shadow mask 95 is welded to a mask frame 96 such that tension is applied in a direction indicated by arrows 9 (a vertical direction, i.e., a Y-axis direction). The shadow mask 95 is provided with a large number of apertures 90, through which electron beams pass and reach a phosphor screen.
In such a tension-type shadow mask 95, the apertures 90 formed in the shadow mask 95 are shaped and arranged as follows. In general, a large number of substantially equi-shaped slot apertures 90 are aligned such that their longitudinal directions correspond to the vertical direction as shown in FIG. 16.
During an operation of the color cathode ray tube, the shadow mask 95 is heated by the electron beams and expands. Although the thermal expansion in the vertical direction is absorbed by the tension applied to the shadow mask 95, the thermal expansion in the horizontal direction is transmitted horizontally via bridges 91, causing so-called doming. For preventing this doming, it is preferable that a vertical pitch of the bridges 91 is large. When the vertical pitch of the bridges 91 is increased, the resultant increase in an aperture area improves brightness of a displayed image. However, there is a problem that the interference between the regularly arranged bridges 91 and horizontal scanning lines causes moiré fringes, deteriorating an image quality.
In order to solve this problem, JP 2001-84918 A discloses a technology in which a pair of vertical sides of each of the apertures 90 in the shadow mask 95 are formed to have protrusions and depressions. FIG. 17 is a schematic view showing the shadow mask 95, a phosphor screen 2a and electron beams 94 that have passed through the apertures 90 of the shadow mask 95 (passed beams 94), seen from an electron gun side.
With this technology, a plurality of protrusions 92 that protrude inward from the pair of vertical sides of the apertures 90 serve as pseudo-bridges. Therefore, even when the vertical pitch of the bridges 91 is extended, it is possible to suppress the generation of moiré fringes caused by the interference between the bridges 91 and the scanning lines. Furthermore, since the number of the bridges 91 can be reduced, the heat is not easily transmitted horizontally via the bridges 91, so that the displacement of the shadow mask apertures owing to doming can be suppressed, thus achieving an effect of preventing color displacement.
Moreover, JP 63(1988)-43241 A suggests that, for preventing breaking of the shadow mask and improving brightness, two kinds of apertures 90a and 90b having different vertical lengths can be aligned in combination as shown in FIG. 18.
However, the above-described conventional technologies respectively have the following problems.
In the technology illustrated in FIG. 17, phosphor lines 12 in the phosphor screen 2a are substantially straight lines, whereas the passed beams 94 have substantially the same shapes as the apertures 90 because the electron beams are blocked by the bridges 91 and the protrusions (pseudo-bridges) 92. Accordingly, non-light-emitting portions are formed in the phosphor lines 12. In general, a higher brightness per unit electric current is desirable in a cathode ray tube, and this can be achieved effectively by removing the non-light-emitting portions. However, with the technology shown in FIG. 17, it has been difficult to increase the brightness because of the bridges 91 and a large number of the protrusions 92. Reducing the vertical width of the bridges 91 can achieve a smaller area of the non-light-emitting portions, but this is problematic in that, owing to a large vertical pitch of the bridges 91, a sufficient mechanical strength cannot be achieved, so that the bridges 91 break easily. Furthermore, reducing the vertical width of the plurality of the protrusions 92 also can achieve a smaller area of the non-light-emitting portions, but there arises a problem that it is difficult to form narrow protrusions 92 with a high dimensional accuracy, so that a variation in color purity is generated.
In addition, a general method for forming the phosphor lines 12 is an exposure method of forming the phosphor lines 12 by exposure using the shadow mask 95 as a mask. In this exposure method, the widths of the phosphor lines to be formed vary with illumination. In the technology illustrated in FIG. 18, since the two apertures 90a and 90b have equal horizontal widths, the illumination of light that has passed through the short aperture 90b, in which a pair of the bridges 91 at both ends in the vertical direction are positioned closer, is smaller than the illumination of light that has passed through the long aperture 90a, in which a pair of the bridges 91 are positioned farther. This causes a difficulty in forming the phosphor lines 12 with equal widths by the exposure method.